Supple sheet material and method of producing same

ABSTRACT

A SUPPLE SHEET MATERIAL SUITABLE AS A BASE MATERIAL FOR THE MANUFACTURE OF ARTIFICIAL LEATHER COMPRISING A NONWOVEN CONDENSED WEB FORMED FROM FIBERS OF SUBSTANTIALLY HYDROPHILIC MATERIAL, E.G., POLYVINYL ALCOHOL, AND A POLYMER FILLER WHICH HAS SUBSTANTIALLY NO ADHESION TO THE FIBERS, E.G., POLYURETHANE. THE FILLER IS HOMOGENEOUSLY DISTRIBUTED THROUGHOUT THE WEB AND COHERES AT SPACES BETWEEN THE FIBERS AND THE SHEET MATERIAL HAS A DENSITY OF AT LEAST 0.5 GRAM PER CUBIC CENTIMETER AT A TOTAL FILLER CONTENT OF NOT MORE THAN 30% BY WEIGHT, BASED ON THE WEIGHT OF THE SHEET MATERIAL. ALSO A METHOD FOR PRODUCING THIS SHEET MATERIAL IS DISCLOSED.

United States Patent 01 fice.

3,597,307 Patented Aug. 3, 1971 US. Cl. 161-170 8 Claims ABSTRACT OF THE DISCLOSURE A supple sheet material suitable as a base material for the manufacture of artificial leather comprising a non woven condensed web formed from fibers of substantially hydrophilic material, e.g., polyvinyl alcohol, and a polymer filler which has substantially no adhesion to the fibers, e.g., polyurethane. The filler is homogeneously distributed throughout the web and coheres at spaces between the fibers and the sheet material has a density of at least 0.5 gram per cubic centimeter at a total filler content of not more than 30% by weight, based on the weight of the sheet material. Also a method for producing this sheet material is disclosed.

This invention relates to a supple sheet material which is particularly suitable to be used as base material for the manufacture of artificial leather. More particularly this invention relates to a supple sheet material which has properties that closely approach or even excel those of dressed animal skin used for the manufacture of natural leather and which is similar to natural leather in that it contains a maximum amount of hydrophilic fibrous material and to a method for producing such sheet material.

Heretofore many attempts have been made to prepare base materials suitable for the manufacture of artificial leather and other substitutes for natural materials. British patent specification No. 804,669 discloses a process for the manufacture of a leather-like sheet material from a web of natural and/ or synthetic fibers. This process comprises the steps of impregnating the web with a solution of a given polyurethane and then subjecting it to an aftertreatment, which may or may not be carried out under pressure at an elevated temperature. The concentration of polyurethane may be considerably reduced if prior to impregnation the Web is treated with a rubbery binder such as a latex of a butadiene-acrylonitrile copolymer. Although incidental mention is made of the use of polyvinyl alcohol fibers or rayon fibers, it is preferred according to this process to use nylon and/or cotton fibers. The resulting web is processed into a leather-like sheet material having a density of approximately 0.4 gram per cubic centimeter; the pick-up on the web of polyurethane, or polyurethane and rubbery binder being on an average 100% by weight or more on the weight of the web.

In US. Pat. 2,962,762 there is described a process for the preparation of polyvinyl alcohol (PVA) fiber sheets, in which the fibers themselves serve as binder. Use is made of PVA fibers which may be crimped or uncrimped and which owing tothe treatment applied (stretching, hot stabilizing and shrinking) are capable of swelling in hot water without their structure being changed and which, when subsequently dried, with or without pressure, will adhere to one another. If desired, a binder may be added in this process. The resulting sheets, which may or may not be subjected to a formalization treatment, have a wide field of application. They may be used, for example, as substitutes for various sorts of paper, felts, table linen, etc.

Netherlands patent application 6511390 describes the manufacture of a polyvinyl alcohol-fiber web which is particularly suitable to serve as a substrate for artificial leather. In this case, the base material is a web or mat comprising water-shrinkable PVA-fibers, which is successively contacted with an aqueous liquid, subjected to a formalization treatment, and so dried that practically no sticking together of the polyvinyl alcohol fibers will occur. The web, when still dry, is preferably subjected to a needle-punching treatment and the aqueous liquid should contain a water-soluble polymer filler which (in a subsequent coagulating bath) is precipitated in the web. In those cases where the resulting web of shrunk and formalized PVA fibers is to be used for the manufacture of synthetic leather, the web may be impregnated and/or coated with a polyurethane composition.

Suitable methods for impregnating a fibrous substrate with a homogeneously distributed and porous polymer filler having no adhesion to the fibers are described in, inter alia, the US. Pat. 3,067,482 and the Netherlands patent application 6511220. The method described in the US. Pat. 3,067,482 comprises applying a polymer solution such as polyurethane in dimethyl formamide (DMF), and treating the impregnated substrate with a liquid miscible with the solvent, for instance water, which causes the polymer to coagulate and which serves to remove the solvent in a washing treatment. According to Netherlands patent application 6511220 the polymer solution is premixed with a small amount of a non-solvent, for instance, water. Thereafter the polymer is co'agulated by a decrease in temperature, and the solvent and the non-solvent are removed. The resulting porous, sheet-like material may be used as base material for the manufacture of artificial leather.

When considering the properties and the structure of the ideal animal skin which one tries to duplicate in the manufacture of synthetic leather to obtain the best possible imitation, it has been found that the synthetic leather materials, produced by the aforementioned processes as well as others that are commercially available, are deficient in one or more respects.

One of the most notable failures of the known products, even of those made from hydrophilic fiber material such as PVA or rayon, has been their low density, as Well as the fact that a relatively high amount of binder and/or filler must be used for improving the density. It should be pointed out that this problem cannot be solved either by simply condensing a fibrous web containing a given binder, because the resulting sheets are then usually boardy and unsuitable to be processed into artificial leather.

It is therefore considered of particular importance to provide a base of highly densified hydrophilic fibrous material having not only the strength but also the water vapor pick-up (which greatly influence the wearing comfort) that very closely approach those of the natural base material.

Advantageously, this invention provides a method of obtaining such a base material.

Thus this invention contemplates a supple sheet material for the manufacture of artificial leather made from hydrophilic fibers, which is characterized in that it has a density of at least 0.5 gram per cubic centimeter at a total filler content of not more than 30%, but preferably of from 10 to 20%, calculated on the weight of the sheet.

The term hydrophilic fibers as used herein is meant to designate fibers which when placed in a moist atmosphere of 20 C. having a relative humidity that changes from 50% to will show a moisture pick-up of at least 7% of their own weight. Such fibers usually have a swelling value of at least 25%, but generally of 40% or higher. The swelling value is defined as the percentage moisture retained by the water-impregnated dry fibers after they have been subjected to a relative centrifugal force of 1,500 grams for minutes.

Advantageously, it has been found that the properties of the supple sheet material of this invention closely approach, or even excel, those of animal skin used for the preparation of leather. This is particularly true of the important properties such as strength, elongation at break, modulus, tear strength, and water vapor pick-up.

It is believed that the unique properties of the supple sheet material are due to the fact that the greater part of the sheet material, namely, at least 70%, is formed by densified hydrophilic fibrous material, and the filler content has been kept as low as practically possible.

Moreover, the moisture-absorption capacity of the sheet material, and consequently its ability to swell and shrink, depending on the moisture content, provides the sheet with a suppleness that greatly contributes to wearing comfort.

It will be understood that in the manufacture of the sheet material of this invention, it is necessary that a proper choice of the base material be made and that a certain combination of process steps be followed.

In this regard the starting materials used for the manufacture of the sheet material of this invention comprise fibers of substantially hydrophilic material such as polyvinyl alcohol (PVA) and regenerated cellulose (rayon), which fibers, while in the form of a condensed nonwoven web or mat are consolidated into a sheet having the desired properties with the aid of a polymer filler that has no adhesion to the fibers.

This invention also therefore contemplates a method for producing a supple sheet material suitable as a base for artificial leather which comprises forming a non-woven web of the substantially hydrophilic fibers, shrinking the fibers to effect initial densification of the web, subjecting the shrunk fibrous web to pressure at an elevated temperature to effect further densification, i.e., to provide a density of at least 0.5 gm./cm. and thereafter introducing a polymeric filler that has substantially no adhesion for the fibers into the highly densified web; the amount of filler not being greater than 30% by weight based on the weight of the sheet material.

More particularly the method of this invention includes the following processing operations in order:

(1) In a dry process a web of hydrophilic fibers, preferably water-shrinkable crimped PVA fibers, is formed and, if necessary, subjected to a needle-punching operation to improve its coherency.

Retractable PVA fibers of this type are known and may be obtained, inter alia, by the methods described in the heretofore described Netherlands patent application 6511390 and the US. Pat. 2,962,762.

In this operation, the fibers preferably each have a denier of from 0.5 to 2.5, and are advantageously out to lengths of about 40 mm.

(2) The fiber web is preferably subjected to a shrinking treatment. In the case of PVA fiber webs shrinking is efiected by treating the webs with water at a temperature below about 70 C.

During this treatment an initial densification of the web takes place. Thereupon, the shrunk web is, when necessary, dried at a temperature not higher than 70 C. During or after the shrinking treatment a binder may be introduced into the web. Suitable binders include dispersions or a variety of solutions such as reactive rubber latices, cyanates, alginates, carboxy methyl cellulose, and the like. It will be appreciated that the binding together of the fibers by a binder need only be of a temporary nature since the binder only serves to maintain the high fiber density obtained in the subsequent processing step or operation until the filler has been introduced. When the shrinking treatment is carried out under conditions such that the fibers themselves will locally and temporarily bond together, then the use of binders is not necessary at all.

In cases where use is made of a binder, it is preferred that the concentration and the amount thereof should be so chosen that the dried web will not contain more than 10% by weight of the binder.

(3) The dried web is now pressed at an elevated temperature-for instance, by passing it between heated rollsto activate the binder and/ or the fibers with simultaneous densification of the web to a value of 0.5 g./cm. or higher. It will be appreciated that the pressing temperature is dependent on the binder applied, if any, but the temperature is usually C. or higher.

The resulting web is formed by a highly densified hydrophilic fibrous material which is kept together at the bonding points throughout the web, and particularly at the fiber crossing points. For the purpose of curing the binder, if applied, it may be useful to subject the web to an after-heating treatment.

It will be understood that if the web is to be processed into synthetic leather, only a small amount of filler in the hesion to the fibers, such as a polyurethane, need still be form of a small amount of a polymer which has no adintroduced into the web.

(4) The polyurethane filler may be introduced by any one of the known methods, as described in, inter alia, the US. Pat. 3,067,482 and the Netherlands patent application 6511220.

The aforedescribed method of impregnating with a solution of polyurethane in a mixture of a solvent, for instance, dimethylformamide, and a small amount of a non-solvent, for instance, water, followed by precipitating and removing the solvent and the non-solvent, is particularly suitable for obtaining a homogeneously distributed and porous filler which has substantially no adhesion to the fibrous material.

Also quite effective is the impregnating method described in the Netherlands patent application 6706867, which relates to the application of an auxiliary substance, such as caprolactam, which will precipitate when cooled down.

The concentration and the amount of the impregnant are invariably so chosen that the total filler content of the web should not be higher than 30% by weight. It is preferred that the amount of filler introduced into the web should not be higher than of from 10 to 20% by weight.

When use is made of a binder which is permanently to form part of the web, the total content of binder and filler must not be higher than 30% by weight, based on the weight of the sheet or Web.

It will be appreciated that only an outline of the processing operations to be employed in the manufacture of the sheet material according to the invention is given above. This outline, of course, does not include variants by which certain properties may be further influenced while the essential characteristics of the sheet are maintained. Exemplary of the variable factors and conditions are the retractability and the degree of crimp of the hydrophilic fibers, the type of binder, the application of aqueous or non-aqueous binder dispersions (if use is made of a non-aqueous dispersion it is, as a rule, desirable that the web should be pre-shrunk in water), the pressure and temperature at which the web is densified and bonded, and the like.

As mentioned before, the supple sheet material of this invention forms an excellent base material to be further processed into artificial leather.

This further processing may simply be carried out by the conventional leather treating methods. For instance, the sheet material may in a conventional manner he fatliquored, hydrophobized, staked, buffed and/or split, and it may be coated by successive napping and spraying. It should be added that the fat-liquoring and hydrophobizing substances also may be introduced during the preceding processing steps. In a final processing step, usually a varnish top coat is applied.

' Use may also be made of different finishing methods known from the synthetic leather technique, more particularly different methods of applying various coatings ineluding, if desired, a binder layer.

The invention and the results obtained thereby are ther described in the following examples:

EXAMPLE I After being given a needle-punching treatment, a web of crimped 1.3 denier polyvinyl alcohol fibers (length 40 mm.) weighing 400 gun/m. is shrunk in water of about 50 C.

The shrunk web is dried and then pressed for 30 seconds between the platens of a press at a temperature of 120 C. and at a pressure of approximately 6 kg./cm. The condensed web has a density of about 0.6 g./cm.

The web is subsequently immersed in a 15 %-solution of a polyurethane (Estane 5707 F1, of B. F. Goodrich Chemical Company) in a mixture of 95 parts dimethyl formamide (DMF) and 5 parts water at 50 C. The excess liquid is removed by passing the web between squeeze rolls after which the web, while being cooled down to 5 C., is washed in water containing a small amount of DMF, followed by drying it at 50-70 C.

The resulting supple sheet material has a density of 0.60 gm./cm. and contains approximately 11% by weight of the polyurethane that has no adhesion to the fibers.

EXAMPLE II A shrunk and dried web produced in the manner in Example I is pressed for 30 seconds between the platens of a press at a temperature of 120 C. and at a pressure of approximately 5 kg./cm. The compressed web has a density of 0.57 gm./cm.

After the additional treatment described in Example I, the sheet material has a density of 0.58 gar/cm. and a polyurethane content of 15% by weight.

EXAMPLE III In this example, the needle-punched web of Example I is impregnated with a %-dispersion of a reactive acrylate rubber (Hycar 2679, of N.V. Chemisehe Industrie AKU-Goodrich) at about 50 C. Owing to this treatment the surface area of the web shrinks by a factor of about 2.

The impregnated web is dried at 45-50 C. and is found to have a binder content of about 8% by weight.

The dried web is then pressed for 30 seconds between the platens of a press to a density of 0.50 gm./crn. at a temperature of 120 C. Following the additional processing operations described in Example I, the web is filled with polyurethane, which is now done by coagulation in DMF-containing water at 20 C. The resulting sheet is found to have a density of 0.53 gm./cm. and a binder content and filler content of 8% by weight and 16% by weight, respectively.

EXAMPLE IV A binder-impregnated web is produced following the procedure described in Example III. The web is slightly squeezed and passed through a /2%aqueous solution of triethyllauryl-ammonium chloride (destabilizer for the rubber dispersion). The web thus treated is again slightly squeezed and then dried at 45-50 C.

Thereupon the dried web is compacted to a density of 0.6 g./cm. and filled as described in Example III. The resulting sheet has a density of 0.60 g./cm. its binder content and filler content being 8% by weight and 10% by weight, respectively.

EXAMPLE V A shrunk and dried web produced as described in Example I is treated at room temperature with the binder dispersion described in Example III, and then pressed to a factor of approximately 2 and dried at 40-45 C.

The dried web is pressed for 30 seconds between the platens of a press to density of 0.75 g./cm. at a temperafurture of 120 C. and then cured for 10 minutes at 120 C.

Thereupon, the web is impregnated with a 20%-aqueous emulsion of a nitrile rubber (Hycar 1562, of N.V. Chemisehe Industrie AKU-Goodricht) which has no adhesion to PVA-fibers. The excess liquid is squeezed out, after which the web is dried at 45-50 C.

The resulting sheet has a density of 0.58 g./cm. a binder content of 8% by weight and contains 18% nitrile rubber filler.

EXAMPLE VI A web of crimped 1.5 denier rayon fibers (length 40 mm.) weighing 600 gm./m. is formed and, after needling, impregnated at room temperature with the binder dispersion described in Example III.

The web is dried at 65-70 C. and is then found to have a binder content of about 14% by weight.

The dried web is subsequently pressed for 30 seconds between the platens of a press to a density of 0.78 gm./cm. at a temperature of 120 C.

The condensed web is filled with polyurethane as described in Example III. The resulting sheet has a density of 0.55 gn1./em. a binder content of 14% by weight and a filler content of 14% by weight.

EXAMPLE VII A shrunk and dried web produced in the manner described in Example I is immersed in a 4%-solution in ethylacetate of a reactive macroisocyanate (a mixture of Imprafix and Impranil which are products of Bayer).

The excess liquid is removed followed by evaporation of the solvent. The dry web is then kept for 24 hours at room temperature.

Thereupon the 'web is pressed for 30 seconds between the platens of a press to a density of 0.7 gm./cm. at a temperature of 120 C. Finally, the condensed web is filled with polyurethane as described in Example III. The resulting sheet has a density of 0.70 g./cm. a binder content of 8% by weight and a filler content of 10% by weight.

EXAMPLE VIII A needle-punched web produced in the manner described in Example I is shrunk in a 2% aqueous alginate solution of 50 C. The excess liquid is removed, after which the web is dried at 4050 C. The binder content is found to be approximately 7% by weight.

The dried web is pressed to a density of 0.6 gm./cm. at a temperature of 120 C.

The web is then filled with polyurethane as described in Example III, after which the resulting sheet is found to have a density of 0.58 g./cm. and a filler content of 14% by weight. The initially introduced binder is found to be completely removed by the washing treatment.

EXAMPLE IX The sheet materials produced in the preceding examples, i.e., Examples I to VIII are further processed into artificial leather and evaluated. In each case the sheet materials provide outstanding artificial leather substrates.

The above examples show that by a proper choice of the starting material and the processing steps there may be obtained a sheet-like product containing a very compact hydrophilic fiber material and a relatively small amount of filler.

It will be understood for wearing comfort, a very important property of leather is its moisture absorption capacity. A measure thereof is, for instance, the water vapor pick-up, expressed in mg./l00 cmfi, in an atmosphere of 20 C. whose relative humidity changes from 50% to 95%.

Whereas a good quality natural leather having a sheet thickness of 1-2 mm. shows a water vapor pick-up of 1,000 or more, the hitherto known artificial leather products of comparable thickness are usually found to show a value as low as -150.

In this respect the product according to the present invention offers a considerable improvement. At a density of 0.5 g./cm. and a filler content of 30% by weight the novel products are found to have a water vapor pick-up of 450 or more, and at a higher density and a lower filler content, which are to be preferred, this value is usually at least as high as 650 (such sheet materials are shown in Examples I, II, IV and VII).

Also :the properties relating to the strength of the material compare with or are superior to those of both natural leather and the known artificial leather products. It has been found that the product according to the present invention generally shows the following values (both in longitudinal and transverse direction): strength 23-35 kg./cm., elongation at break 7080%, modulus at 10% elongation 5-6 kg./cm., mean tear strength 8 kg.

It should be added that the term fibers as used herein is meant to designate staple fibers and continuous filaments. Hydrophilic material in the form of grindings or other particulate matter may also be introduced into the fibrous web.

Moreover it will be appreciated that if desired, the product according to the invention may be made to contain a small amount of non-hydrophilic fibers such as those of nylon, polyester or polypropylene. However, in view of the envisaged hydrophilic character of the product, the amount of non-hydrophilic fibers to be admixed must be kept small, and, in general, is less than 5%.

What is claimed is:

1. A supple sheet material suitable to be used as base material for the manufacture of artificial leather which comprises a non-woven condensed web formed from fibrous material consisting essentially of substantially hydrophilic polymeric fibers, and a polymer fillter which has substantially no adhesion to said fibers, said hydrophilic polymeric fibers having a moisture pick-up of at least 7% of the weight of said fibers when placed in a moist atmosphere with a temperature of 20 C. and a relative humidity that changes from 50% to 95%, said filler being homogeneously distributed throughout said web and cohering at spaces between said fibers, and said sheet material having a density of at least 0.5 gram per cubic centimeter and a total filler content of not more than 30% by weight based on the weight of the sheet material.

2. The supple sheet material of claim 1 in which filler content is in an amount of from 10 to 20% by weight. 3. The supple sheet material of claim 1 in which the filler is a polyurethane.

4. The supple sheet material of claim 1, in which the hydrophilic fibers are selected from the group consisting of polyvinyl alcohol and regenerated cellulose fibers.

5. The supple sheet material of claim 1 further comprising a binder by which said fibers are bonded together at randomly distributed fiber crossing points; the total amount of binder and filler not being more than 30% by weight, based on the weight of the sheet material.

6. The supple sheet material of claim 5 in which said sheet material contains from 5 to 10% by weight of the binder and from 10 to 20% by weight of the filler.

7. Artificial leather manufactured from the supple sheet material of claim 1.

8. The supple sheet material of claim 1 in which a sheet having a thickness of 1 to 2 mm. exhibits a water vapor pick-up of at least 450 mg./ cm. when placed in said moist atmosphere.

References Cited MORRIS SUSSMAN,

US. Cl. X.R.

Primary Examiner 

